This invention relates to an oil type damper for braking the opening or closing motion of a lid or the like.
In an automobile, for example, a push given to a button causes a retractable ash tray to slide out to its service position or the closed lid of a glove compartment to pop out and provide access to the interior of the glove compartment. In a video cassette tape recorder, a similar push to a button causes a cassette tape holder in its concealed position to rise and come into sight out of the housing of the video set. For the purpose of braking the aforementioned sudden motion of a closed component in a retractable device, there has been proposed a frictional damper (Japanese Patent Application Disclosure No. Sho 55(1980)-75805) or an oil type damper (U.S. Pat. No. 4,342,135), for example. The frictional damper entails problems such as difficulty in providing a smooth braking motion, an awkward sensation to the user's finger placed on the button, and gradual degradation of performance during the course of prolonged service. Although the oil type damper does not encounter the problems inherent in the frictional damper, it nevertheless suffers from a peculiar problem of its own, namely, a problem arising from the use of oil. In the oil type damper, silicone grease having high viscous resistance is generally used as the oil in an amount of about 0.15 g. The silicone grease has a very high thermal expansion coefficient and its volume increases at the rate of 0.1% per degree Centigrade. If the temperature of the oil type damper in service happens to rise, therefore, the silicone grease may be expanded so much as to leak through the fine gap between the shank of a rotary member passing through the wall of the housing and the hole in the wall or through the fine gap between the case constituting the housing and the lid formed in the case. Once this leakage occurs, the oil type damper may possibly be deprived of its function totally or partially owing to the resultant shortage of silicone grease. To avoid this trouble, there has been developed a device for securing a tight seal between the case and the lid by the use of an ultrasonic welder. Since the case and the lid are both made of plastic, they are expanded in volume because of the elevation of temperature by the heat from the welder, with the result that the space left for accommodating the oil is decreased so much as to squeeze out the oil. Thus, this device fails to prevent the leakage of oil after all.